For a highly-sensitive magnetometer which is operable at a room temperature, a diamond crystal including nitrogen-vacancy centers (NV centers) has been proposed (see, for example, D. Sage et al., “Efficient photon detection from color centers in a diamond optical waveguide” PHYSICAL REVIEW B 85, 121202(R) (2012) (Non-Patent Document 1)).
The Non-Patent Document 1 discloses that, by detecting fluorescence generated from a diamond crystal while irradiating the diamond crystal with excitation light of a green laser and a microwave pulse train in a controlled cycle, an alternating-current magnetic field corresponding to the cycle can be measured.
It further discloses that the sensitivity improvement can be achieved by collecting also a part of the fluorescence radiated from a periphery of the diamond crystal in various directions in addition to the commonly-performed detection of a part of the fluorescence reflected in an incidence direction of excitation light.